Britain vs non-U

Britain is a noun but non-U is not a noun.

Word	Noun	Adjective	Verb	Adverb
Britain	Yes	No	No	No
non-U	No	Yes	No	No

Britain (noun)	non-U (noun)
a monarchy in northwestern Europe occupying most of the British Isles; divided into England and Scotland and Wales and Northern Ireland; `Great Britain' is often used loosely to refer to the United Kingdom

Britain (adjective)	non-U (adjective)
	not characteristic of the upper classes especially in language use